1. Field of the Invention
Preferred aspects of the present invention relate to rotating reverse osmosis (RO) filtration, wherein filtrate flux is enhanced by creation of shear and Taylor vortices in the coaxial gap between a RO membrane and a cylindrical wall of the filtration device.
2. Description of the Related Art
One of the most limiting problems in filtration is filter clogging, scientifically described as “concentration polarization.” As a result of the selective permeability properties of the filter, the filtered material that cannot pass through the filter becomes concentrated on the surface of the filter. This phenomenon is clearly illustrated in the case of a “dead-end” filter, such as a coffee filter. During the course of the filtration process, the filtered material (coffee grounds) building up on the filter creates flow resistance to the filtrate, the fluid (coffee) which can pass through the filter. Consequently, filtrate flux is reduced and filtration performance diminishes.
Various solutions to the problem of concentration polarization have been suggested. These include: increasing the fluid velocity and/or pressure (see e.g., Merin et al., (1980) J. Food Proc. Pres. 4(3):183-198); creating turbulence in the feed channels (Blatt et al., Membrane Science and Technology, Plenum Press, New York, 1970, pp. 47-97); pulsing the feed flow over the filter (Kennedy et al., (1974) Chem. Eng. Sci. 29:1927-1931); designing flow paths to create tangential flow and/or Dean vortices (Chung et al., (1993) J. Memb. Sci. 81:151-162); and using rotating filtration to create Taylor vortices (see e.g., Lee and Lueptow (2001) J. Memb. Sci. 192:129-143 and U.S. Pat. Nos. 5,194,145, 4,675,106, 4,753,729, 4,816,151, 5,034,135, 4,740,331, 4,670,176, and 5,738,792, all of which are incorporated herein in their entirety by reference thereto).
Taylor vortices are induced in the gap between coaxially arranged cylindrical members when the inner member is rotated relative to the outer member. Taylor-Couette filtration devices generate strong vorticity as a result of centrifugal flow instability (“Taylor instability”), which serves to mix the filtered material concentrated along the filter back into the fluid to be processed. Typically, a cylindrical filter is rotated within a stationary outer housing. It has been observed that membrane fouling due to concentration polarization is very slow compared to dead-end or tangential filtration. Indeed, filtration performance may be improved by approximately one hundred fold.
The use of Taylor vortices in rotating filtration devices has been applied to separation of plasma from whole blood (see e.g., U.S. Pat. No. 5,034,135). For this application, the separator had to be inexpensive and disposable for one-time patient use. Further, these separators only had to operate for relatively short periods of time (e.g., about 45 minutes). Moreover, the separator was sized to accept the flow rate of blood that could reliably be collected from a donor (e.g., about 100 ml/minute). This technology provided a significant improvement to the blood processing industry. The advantages and improved filtration performance seen with rotating filtration systems (Taylor vortices) have not been widely exploited in other areas of commercial fluid separation.
In commercial blood separators, a fluid seal and mechanical bearings prevent the separated plasma from remixing with the concentrated blood cells. Pressure drives the plasma through the seal and mechanical bearings and into a tubing port that leads to a collection container. The rotor spins on an axis defined by two shaft bearings, one on either end. Spinning is induced by a rotating magnetic field and a magnetic coupling. A motor with permanent magnets fixed to its rotor generates the rotating magnetic field. While this design is appropriate for a disposable blood separator, it is not well adapted for long-term operation. First, the design adds a rotational drive motor to any filtration system, beyond the pump(s) needed for fluid feed and collection. Further, the seals are likely to wear out if the rotor is spun at 3600 rpm for prolonged periods. Likewise, the bearings that support the rotor are also likely to wear out. Use of seals and bearings adapted for continuous long-term use (like those used conventional pumps) are expensive and suffer from reliability concerns.
One other fluid separation technology, reverse osmosis (RO) membrane filtration, is well suited for removal of dissolved ions, proteins, and organic chemicals, which are difficult to remove using conventional filtration methods. Further, RO membrane systems are regenerable, thereby providing long term membrane service, requiring replacement only 1-2 times per year in commercial membrane plants. Moreover, because RO is an absolute filtration method, its treatment efficiency and performance are stable and predictable (Lee and Lueptow (2001) Reverse osmosis filtration for space mission wastewater: membrane properties and operating conditions. J. Memb. Sci. 182:77-90). However, membrane fouling due to concentration polarization is still a problem in conventional RO filtration.
Lee and Lueptow recently published a study that suggests that rotating filtration devices that use Taylor vortices to reduce concentration polarization may be used to enhance filtrate flux through reverse osmosis (RO) membranes (Lee and Lueptow (2001) Rotating reverse osmosis: a dynamic model for flux and rejection. J. Memb. Sci. 192:129-143). Unfortunately, existing Taylor-Couette systems/devices, such as those discussed above with respect to blood separation, are poorly suited for large scale commercial applications where long-term continuous operation is desirable. Consequently, a need exists for energy efficient; rotating membrane filtration systems/devices, compatible with reverse osmosis membranes, adapted to long-term continuous use and scalable for commercial separation applications.